Bright products obtained by continuous casting

ABSTRACT

A bright composite sheet product including a roll cast core and a clad material bonded to the core. The core is an aluminum alloy of the AA series 1XXX, 3XXX, 5XXX, or 8XXX, and the clad material contains at least 99.5 wt. % aluminum. The composite sheet products is particularly suited for use in lighting fixtures.

FIELD OF THE INVENTION

[0001] The present invention relates to clad metallurgical products andto their manufacture, more particularly, bright products produced bylaminating a high purity aluminum alloy cladding onto a continuouslycast core.

BACKGROUND OF THE INVENTION

[0002] Clad aluminum alloy materials for brazing applications having analuminum alloy core material and aluminum alloy filler material as asurface skin is well known. This material is most commonly formed bysuperimposing an aluminum alloy plate which serves as a skin materialover an aluminum alloy cast ingot which serves as the core material. Thefiller plate can be applied to one or both surfaces of the ingot. Thecomposite then undergoes hot rolling to bond the two layers together tothicknesses of about 3 to about 10 mm. The clad material is thensubsequently cold rolled down to thicknesses of about 0.075 to about 3mm to obtain brazing sheet. The sheet may be annealed to aid in rollingefficiency or to achieve desired final mechanical properties.

[0003] Aluminum brazing sheet is used in a variety of thicknesses,depending on the application. For instance, brazing sheet used in airconditioners have thicknesses ranging from about 1.5 to about 2 mm whenbeing used for support purposes, from 0.5 to 0.65 mm as evaporatorplates, from about 0.3 to about 0.4 mm as tube stock (sheet materialformed into tubes for carrying fluids, i.e. gases, vapors, and liquids)and from about 0.075 to about 0.15 mm as fin stock (sheet material fromwhich, for instance, radiator fins are made).

[0004] More recently, clad brazing sheet has been made by continuouslycasting the core alloy between a pair of rolls while laminating a sheet(band) of the cladding alloy to the core alloy at the rolls in a systemdescribed in U.S. Pat. No. 5,476,725, incorporated herein by reference.

[0005] Only certain clad aluminum sheet products have been produced bythis method of continuously laminating a cladding band onto a castingcore alloy, namely, aluminum brazing sheet. The alloy of the core of abrazing sheet typically is produced from an alloy of the 3XXX or 6XXXAluminum Association (AA) series and the clad layer generally is an AA4XXX series alloy. Consequently, the clad layer of aluminum brazingsheet conventionally has a lower solidus temperature (the temperature atwhich some melting of the material first occurs) than the solidustemperature of the core alloy.

[0006] The production of other clad aluminum sheet products,particularly bright aluminum sheet products, is accomplished via theconventional method of roll bonding a clad layer onto a cast ingot of acore alloy. Bright sheet products require that the clad layer beproduced from a high purity aluminum alloy (at least 99.5 wt. %aluminum) which has a higher solidus temperature than the solidustemperature of the core. The roll bonded high purity aluminum clad layermay be further finished by electrobrightening and anodizing to achieve adesired reflective finish to the sheet. Bright products should have aminimum of surface defects (dents, scratches or spots) with a highdegree of surface brightness and uniformity to ensure good and uniformoptical properties of the sheet after anodizing. These properties haveheretofore only been achieved by roll bonding the high purity aluminumclad layer to a cast ingot. The cladding ratio (the ratio of thethickness of the clad layer to the total sheet thickness) typically isabout 5 to 20%. Uniformity in the cladding ratio along a roll bondedbright sheet in the transverse and longitudinal directions haspreviously been poor, particularly at higher cladding ratios. Whilecladding ratios of 5% can be produced with differences of 1% along aroll bonded bright sheet, the degree of nonuniformity increases withgreater cladding ratios and reaches 3% nonuniformity for cladding ratiosof 20%.

[0007] Hence, a need remains for a method of producing bright sheetproducts in a continuous process which meets the surface qualityrequirements for bright sheet product with high uniformity in claddingratio.

SUMMARY OF THE INVENTION

[0008] This need is met by the clad bright sheet product of the presentinvention manufactured without hot rolling thick ingot with a thickcladding down to a desired gauge of the sheet. Specifically, the presentinvention includes a roll cast clad bright product having a roll castcore of a first aluminum alloy and a clad layer of a second aluminumalloy bonded to one side of the core, the second aluminum alloycontaining at least about 99.5 wt. % aluminum.

[0009] The first alloy preferably is an alloy of the AA series 1XXX,3XXX, 5XXX or 8XXX. The second alloy preferably contains at least about99.8 wt. % aluminum and may be one of AA alloys 1085, 1090, 1095, 1198and 1199. The solidus temperature of the first aluminum alloy is lowerthan the solidus temperature of the second alloy.

[0010] The bright product of the present invention may be less thanabout 2.5 mm thick, preferably about 0.3-1 mm thick. The clad layerpreferably accounts for about 5 to about 20% of the thickness of thebright product, more preferably about 10 to about 15% of the thicknessof the bright product.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A complete understanding of the invention will be obtained fromthe following description when taken in connection with the accompanyingdrawing figures wherein like reference characters identify like partsthroughout.

[0012]FIG. 1 is a schematic cross section of the sheet product of thepresent invention;

[0013]FIG. 2 is a schematic flow path of a method of producing the sheetproduct of the present invention;

[0014]FIGS. 3a and 3 b are photomicrographs of cross sections of thecontinuous clad product of Example 1 following two hour heat treatmentsof 375° C. and 500° C., respectively;

[0015]FIGS. 4a and 4 b are photomicrographs of the surface of theproduct of Example 1 after bright rolling at magnifications of 32 timesand 120 times, respectively; and

[0016]FIGS. 5a and 5 b are photomicrographs of the surface of theproduct of Example 1 after electrobrightening and anodizing atmagnifications of 32 times and 120 times, respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0017] For purposes of the description hereinafter, it is to beunderstood that the invention may assume various alternative variationsand step sequences, except where expressly specified to the contrary. Itis also to be understood that the specific devices and processesillustrated in the attached drawings, and described in the followingspecification, are simply exemplary embodiments of the invention. Hence,specific dimensions and other physical characteristics related to theembodiments disclosed herein are not to be considered as limiting.

[0018] As shown in FIG. 1, the present invention includes bright sheetproduct S having a core layer CO and a cladding layer CL on one sidethereof produced by laminating the cladding alloy onto the core materialin a roll caster. The material of the core layer CO preferably is afirst aluminum alloy of the AA series 1XXX, 3XXX, 5XXX or 8XXX.Particularly preferred alloys for the core layer CO are alloy AA 1050and alloy AA 1200. The core alloy of AA 1050 may further include 0.7 wt.% manganese. For obtaining a bright product, the material of thecladding layer CL should be a second aluminum alloy with at least about99.5 wt. % aluminum, more preferably at least about 99.8 wt. % aluminum.Particularly preferred alloys for the clad layer CL are AA alloys 1085,1090, 1095, 1198 and 1199. The solidus temperature of the first aluminumalloy preferably is lower than the solidus temperature of the secondalloy.

[0019] The bright sheet product S of the present invention preferably isless than about 2.5 mm thick, more preferably about 0.3 to about 1 mmthick. Preferably, the cladding ratio is about 5 to 20%, more preferablyabout 10 to about 15%. The cladding ratio varies along a strip producedaccording to the present invention by less than about 1%.

[0020] The bright sheet of the present invention is produced in theprocess 2 schematically presented in FIG. 2. The clad layer CL isproduced by providing a melt of the clad alloy in step 4 and casting theclad alloy in step 6 into an ingot in a specialized furnace to avoidcontamination of detrimental elements such as iron and silicon and tomaintain purity. Strict control of the casting parameters in the castingoperation is desirable in order to achieve a uniform grain size andstructure. Each side of the cast clad ingot is scalped in step 8 (e.g.,by about 12 mm) to remove surface oxidation and cortical zone and thescalped ingot is hot rolled (e.g., to about 8 mm) with edge trimming instep 10. The rolled product is further reduced by cold rolling (e.g., toabout 0.5 to about 2 mm) in step 12 and preferably is coiled into a coil14.

[0021] In step 24, a melt of the core alloy is charged from a meltingfurnace 26 into a holding furnace 28, passes through a tundish 30 and isdelivered via a nozzle 32 to a roll bite between a pair of rolls 34 and36. Preferably, a strip of the core alloy alone is first produced tostabilize the casting process. The coil 14 of the clad layer CL isunwound and a band 38 of the clad layer CL is introduced into the rollbite in close contact with one of the rolls 34 and 36. A strip 40exiting the rolls 34 and 36 includes the clad layer CL laminated ontothe core layer CO. The temperature of the melt of the core alloy, thecooling by the rolls 34 and 36, the tension in the strip 40 and thetension in the band 38 of the clad layer CL from the coil 14 areadjusted to stabilize the combined cladding and casting process. Inorder to obtain the desired thickness of the strip, desired claddingratio and good adhesion between the clad layer CL and the core layer CO,it is preferred that the core molten metal temperature in the tundish 30is maintained within a close range e.g., of about 680 to 720° C., thespeed of the rolls 34 and 36 is about 0.9 to 1.2 m/min, the castingspeed is about 1.1 to 1.3 m/min, the tension in the band 38 is about 30to 50 Mpa, and the speed of the band 38 is about 0.7 to 1.0 m/min. Thestrip 40 preferably is about 5 to about 6 mm thick with a cladding ratioof about 7 to 15%.

[0022] In step 42, the strip 40 is cold rolled in one or more passesusing rolls having a roughness of about A-40 to an intermediatethickness, e.g. of about 0.65 mm, and in step 44 the strip 40 is coldrolled in one or more passes with finishing rolls (roughness of aboutA-25) to achieve a final thickness, e.g. of about 0.45 mm. Preferably,the roughness of the rolls is progressively decreased as the material isrolled down. In step 46, the sheet 40 may be bright rolled in one ormore passes using rolls with a very low roughness, i.e. rolls withmirror-like surfaces with a roughness Ra of about 0 microns. The finalproduct may be about 0.4 mm thick with a gloss of greater than 75,preferably greater than 80 measured at 20 degrees. The final product maybe further subjected to an electrobrightening and anodizing treatment inorder to increase the brightness and the corrosion resistance so thatthe sheet is suitable for lighting applications.

[0023] The sheet produced according to the present invention has severaladvantages over the roll bonded products of the prior art. Theseadvantages include higher mechanical properties, lower production costsby avoiding hot rolling passes and minimizing the number of cold rollingpasses, and improved uniformity of cladding ratio in length and width.The continuous cladding process of the present invention makes changesin the composition of the core and clad alloys to produce differentalloy combinations easier to accomplish.

[0024] Although the invention has been described generally above, thefollowing example gives additional illustration of the product andprocess steps typical of the present invention.

EXAMPLE

[0025] Alloy AA 1200 was maintained in a tundish at about 700° C. Themelt was continuously roll cast at about 1.2 m/min at a roll speed ofabout 1.1 m/min. A 1mm thick strip of AA 1085 was continuously bonded tothe cast alloy of AA 1200 at a reduction factor of about 0.6 accordingto the process described above. The chemical composition of the corealloy and the clad alloy were as listed in Table 1. TABLE 1 Alloy Si FeCu Mn Mg Cr Ni Zn Ga V Ti B Clad .049 .034 <.001 .002 .005 <.001 .003.007 .002 .026 .026 .0011 Core .03 .10 .066 .50 .003 <.001 .003 .017.015 .006 .007 .0006

[0026] The strip exiting the caster was about 5 mm thick with a claddingratio of about 10%. The cladding ratio at the center of the strip wasdetermined to be essentially the same as the cladding ratio at the edgesof the strip. A laboratory sized portion of the strip was rolled down toabout 0.4 mm according to the process described above. The laboratoryrolled strip was heat treated to determine resistance to blistering at375° C. for 2 hours and at 500° C. for 2 hours.

[0027]FIGS. 3a and 3 b are transverse cross-sectional views of the stripfollowing the thermal treatment at 375° for 2 hours and 500° for 2hours, respectively. Both samples show excellent adhesion of thecladding layer CL (liner) to the core layer CO (core).

[0028] Samples of the strip were bright rolled. FIGS. 4a and 4 b showthe surface of the strip following bright rolling at 32 times and 120times magnification, respectively. The strip exhibits high surfacequality with minimal defects.

[0029] The bright rolled strip was further subjected toelectrobrightening and anodizing treatments. In the electrobrighteningtreatment, the strip was submerged in a bath of phosphoric acid and 45%sulfuric acid at 58-60° C. for 75 seconds at a current density of 10-15A/dm² and a voltage of about 20-23 volts. In the anodizing treatment,the strip was submerged in a bath of 15% sulfuric acid at 20° C. for 5minutes at a current density of 0.8-1.0 A/dm² and a voltage of about10-12 volts. The resulting bright surface is shown in FIGS. 5a and 5 bat 32 times and 120 times magnification, respectively.

[0030] It will be readily appreciated by those skilled in the art thatmodifications may be made to the invention without departing from theconcepts disclosed in the foregoing description. Such modifications areto be considered as included within the following claims unless theclaims, by their language, expressly state otherwise. Accordingly, theparticular embodiments described in detail herein are illustrative onlyand are not limiting to the scope of the invention which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

1. A roll cast clad bright product comprising: a roll cast corecomprising a first aluminum alloy, said first aluminum alloy beingselected form the group consisting of AA series 1XXX, 3XXX, 5XXX and8XXX; and a clad layer comprising a second aluminum alloy bonded to oneside of said core, said second aluminum alloy comprising at least about99.5 wt. % aluminum.
 2. The bright product of claim 1, wherein saidfirst alloy is selected from the group consisting of AA 1050 and AA1200.
 3. The bright product of claim 2, wherein said first alloycomprises an alloy of AA 1050 and an additional about 0.7 wt. %manganese.
 4. The bright product of claim 1, wherein said second alloycomprises at least about 99.8 wt. % aluminum.
 5. The bright product ofclaim 4, wherein said second alloy is selected from the group consistingof AA alloys 1085, 1090, 1095, 1198 and
 1199. 6. The bright product ofclaim 1, wherein said bright product is less than about 2.5 mm thick. 7.The bright product of claim 6, wherein said bright product is about 0.3to about 1 mm thick.
 8. The bright product of claim 1, wherein said cladlayer comprises about 5 to about 20% of the thickness of said brightproduct.
 9. The bright product of claim 8, wherein said clad layercomprises about 10 to about 15% of the thickness of said bright product.10. A roll cast clad bright product comprising: a roll cast corecomprising a first aluminum alloy; and a clad layer comprising a secondaluminum alloy bonded to one side of said core, wherein the solidustemperature of said first aluminum alloy is lower than the solidustemperature of said second alloy.
 11. The bright product of claim 10,wherein said bright product is less than about 2.5 mm thick.
 12. Thebright product of claim 10, wherein said first aluminum alloy isselected from the group consisting of AA series 1XXX, 3XXX, 5XXX and8XXX and said second alloy comprises at least about 99.5 wt. % aluminum.13. A roll cast clad bright product consisting essentially of: a rollcast core comprising a first aluminum alloy; and a clad layer comprisinga second aluminum alloy bonded to one side of said core, said secondaluminum alloy comprising at least about 99.5 wt. % aluminum.
 14. Thebright product of claim 13, wherein said first alloy is selected fromthe group consisting of AA series 1XXX, 3XXX, 5XXX and 8XXX.
 15. Thebright product of claim 14, wherein said first alloy is selected fromthe group consisting of AA 1050 and AA
 1200. 16. The bright product ofclaim 13, wherein said second alloy comprises at least about 99.8 wt. %aluminum.
 17. The bright product of claim 16, wherein said second alloyis selected from the group consisting of AA alloys 1085, 1090, 1095,1198 and
 1199. 18. The bright product of claim 13, wherein said brightproduct is less than about 2.5 mm thick.
 19. The bright product of claim18, wherein said clad layer comprises about 5 to about 20% of thethickness of said bright product.
 20. The bright product of claim 19,wherein said clad layer comprises about 10 to about 15% of the thicknessof said bright product.